Although a superconductor permits a large current to pass with no resistance in a superconductive state, when a current larger than a particular value (critical current) passes, it generates an electrical resistance. When the current is further increased, the temperature of the superconductor rises owing to generated heat so that the superconductor goes into a normal conductive state, thereby generating a large electrical resistance. By making the use of such a feature of the superconductor, a superconducting fault-current limiter has been employed which has no resistance during a normal operation condition and generates a large resistance in the short-circuiting accident of an electric power system, thereby suppressing an increase in the accident current.
A serious problem in promoting dereguration in power systems is an increase in the fault-current at the short-circuiting accident due to the connection of new distributed power sources. The most hopeful countermeasure therefor is introduction of a fault-current limiter which has a low impedance during normal operating conditions and develop a high impedance in a system accident, thereby suppressing the accident current. The introduction of the fault-current limiter provides merits of reduction in the specification of the accident current of the distributed power sources and also contributes to cost reduction of the distributed power sources and improvement of maintenance of facilities. From the point of view of promoting power dereguration, the social demand for realization of the fault-current limiter at low cost and with high reliability is very high. Assuming that the fault-current limiter is introduced in a power distributing system, a superconducting thin-film fault-current limiter employing a superconducting thin film having large area is excellent in various points such as being compact, instantaneously responding to an overcurrent, generating a small AC loss during the normal operating conditions, etc. and therefore, it is supposedly most excellent from the point of views of reliability, performance, volume and extension to a large capacity.
The superconducting thin film fault-current limiter has a thin film current-limiting element operating at a liquid nitrogen temperature (66 to 77.3K), connected in series with a power system. In this fault-current limiter, as the current in the short-circuiting accident increases, the thin film changes from the superconductive state (S) to the normal conductive state, so that the system current is suppressed by a normal resistance. This fault-current limiter is also called an SN transition-type resistive fault-current limiter. Conventionally, a superconducting thin film having large area has been employed in which the thin film of a high temperature superconducting oxide such as YBa2Cu3O7 (hereinafter referred to as YBCO) is formed on an insulator substrate such as a sapphire substrate (single-crystal alumina substrate). However, since the superconducting thin film is expensive, it has been demanded to decrease the area of the superconducting thin film employed as the fault-current limiting element to the utmost, thereby reducing the cost.
The superconducting thin-film fault-current limiting element limits the current by generating a resistive voltage V in an accident. In this case, when the voltage which can be generated (applied) for the unit length of the thin-film current-limiting element (sharing electric field) is high, the length of the element can be correspondingly shortened, whereby the area required for the superconducting thin film can be reduced. However, since the quantity of heat generated by the thin-film current-limiting element during the current-limiting operation can be expressed as P=V2/R, increase in the sharing electric field leads to an increase in the quantity of generated heat. The thin-film current-limiting element is generally designed so that the temperature of the superconducting thin film does not exceed room temperature for a rated fault duration (e.g. 0.1 sec). Therefore, in order to improve the sharing electric field, it is necessary to suppress the increase in the quantity of generated heat of the superconducting thin film during the current-limiting operation, or suppress a temperature rise by increasing the heat capacity of the superconducting thin film. However, the latter increases the volume of the expensive insulating substrate and so will lead to cost increase. Thus, in order to improve the sharing electric field, it is desirable to suppress the increase in the quantity of generated heat by increasing the resistance R when the superconductive path goes into a normal conductive state.
The resistance of the superconductive path can be increased by connecting only superconducting thin films in series or in parallel. When the large-area superconducting thin-film employed for the fault-current limiting element is very uniform and the transition to the normal conductive state is done nearly simultaneously over the entire area, such a configuration can be realized. There is a report of the experiment in a laboratory using the thin film with a low critical current density (see Non-Patent Reference 1). However, in actual use, since the thin film with a high critical current density is adopted, there is the problem of a “hot spot” as described in the following paragraph. In order to overcome such a problem, a shunting resistor must be connected in parallel with the superconducting film as shown in FIG. 5.
The superconducting thin film has variation in the local critical current density. Therefore, at the initial time of current-limiting immediately after the accident, the region with the low critical current density first goes into the normal conductive state but the entire region does not go into the normal state. As a result, a large current continues to flow. In the case that the diffusion of heat generated at the region that has become the normal state is slow, the temperature of this region will locally abruptly rise so that the thin film is burned. A conventional measure for preventing such a hot spot phenomenon is to deposit a normal conductive metal such as gold or silver on the superconducting thin film, which is used as a shunting layer at the time of transition to the normal state (protective layer for preventing burning) (see Non-Patent Reference 2). However, addition of such metallic shunting layer greatly reduces the electrical resistance of the superconductive path and increases the heat generation during the current-limiting operation. For this reason, the sharing electric field must be reduced. As a result, in order to acquire the required current-limiting capacity, the element length must be increased and a large quantity of the expensive superconducting thin film must be employed. This is a serious obstacle to actual use.
In order to reduce the area of the superconducting thin film to be employed to the utmost, there is proposed a technique in which the metallic shunting layer is not deposited on the superconducting thin film but a metallic thin-film shunting layer is formed on another ceramic substrate having a high thermal conductivity and connected to the superconducting thin film through an indium plate (see Patent Reference 1, Non-Patent Reference 3 and Non-Patent Reference 4). In this technique, the heat generated during the current-limiting operation is absorbed by the ceramic substrate different from the superconducting thin film. Therefore, by increasing its heat capacity, the temperature rise in the element can be suppressed and accordingly, the sharing electric field of the element can be increased. For example, the area of the superconducting thin film formed on a sapphire substrate necessary for the current-limiting element in a class of 6.6 kV/2 kA can be reduced to about 1/30 of the conventional element. This supposedly leads to great cost reduction. However, according to this technique, it is necessary to use a large quantity of a highly thermal conductive ceramic substrate such as aluminum nitride and indium plate. Since these materials are expensive, there has been a limit in cost reduction.    Non-Patent Reference 1: A. Heinrich, R. Semerad, H. Kinder, H. Mosebach and M. Lindmayer, “Fault current limiting properties of YBCO-films on sapphire substrates”, IEEE Trans. Appl. Supercond. 9 (1999) 660-663    Non-Patent Reference 2: B. Gromoll, G. Ries, W. Schmidt, H.-P. Kraemer, B. Seebacher, B. Utz, R. Nies, H.-W. Neumueller, E. Baltzer, S. Fischer and B. Heismann, “Resistive fault current limiters with YBCO films-100 kVA functional model”, IEEE Trans. Appl. Supercond. 9 (1999) 656-659    Non-Patent Reference 3: H. Kubota, Y. K. Arai, M. Yamazaki, H. Yoshino and H. Nagamura, “A new model of fault current limiter using YBCO thin film”, IEEE Trans. Appl. Supercond. 9 (1999) 1365-1368    Non-Patent Reference 4: Kubota, Kudo, Yoshino, Wachi, “Design of a fault current limiter using YBCO thin film”, Abstract of the 64th Meeting on Cryogenics and Superconductivity, (2001) 166    Patent Reference 1: Japanese Patent No. 2954124